Pressure-sensitive adhesives in aqueous phase have experienced considerable development since the beginning of the 1970s, to replace the products in solution form, constrained by the new environmental regulations and increases in solvent prices.
Pressure-sensitive adhesives must have the following general characteristics:
be capable of deforming under low pressures to obtain immediate wetting of the surface; PA1 possess sufficient viscoelastic properties to retain a permanent position after removal of the applied force. PA1 (A) 40 to 95%, preferably 50 to 65%, by weight of at least one (meth)acrylic or vinyl monomer capable of producing a homopolymer which has a glass transition temperature lower than or equal to -40.degree. C.; PA1 (B) 2 to 50%, preferably 25 to 45%, by weight of at least one (meth)acrylic or vinyl monomer capable of producing a homopolymer which has a glass transition temperature higher than or equal to 0.degree. C.; PA1 (C) 0.5 to 6%, preferably 1 to 3%, by weight of at least one carboxylic (meth)acrylic monomer; PA1 (D) 0 to 5%, preferably 0 to 3%, by weight of at least one (meth)acrylic monomer ethoxylated with 1 to 20 moles, in particular 1 to 5 moles, of ethylene oxide; PA1 (E) 0.05 to 1%, preferably 0.1 to 0.5%, by weight of at least one (meth)acrylic or vinyl monomer containing a ureido group; and PA1 (F) 0 to 2%, preferably 0.5 to 1.5%, by weight of at least one acrylic or vinyl monomer carrying a sulphonate functional group, PA1 2-Ethylhexyl acrylate . . . 56 parts PA1 Methyl acrylate . . . 41.5 parts PA1 Acrylic acid . . . 2.5 parts PA1 Demineralized water . . . 37.8 parts PA1 n-Dodecyl mercaptan . . . 0.1 parts PA1 Nonylphenol polyoxyethylenated with 10 moles of ethylene oxide (Synthopon.RTM.) . . . 2.39 parts PA1 Sodium lauryl sulphate (Texapon.RTM.) . . . 0.81 parts PA1 Brookfield viscosity at 29.degree. C.: 480 mPa s PA1 Mean particle size: 214 nm PA1 Solids content: 55.2%. PA1 2-Ethylhexyl acrylate . . . 56 parts PA1 Methyl acrylate . . . 38.5 parts PA1 Acrylic acid . . . 2.5 parts PA1 Demineralized water . . . 37.8 parts PA1 n-Dodecyl mercaptan . . . 0.1 parts PA1 Nonylphenol polyoxyethylenated with 10 moles of ethylene oxide (Synthopon.RTM.) . . . 2.39 parts PA1 Sodium lauryl sulphate (Texapon.RTM.) . . . 0.81 parts PA1 Ethyltriglycol methacrylate . . . 3 parts PA1 Brookfield viscosity at 230.degree. C.: 930 mPa s PA1 Mean particle size: 130 nm PA1 Solids content: 56.1%. PA1 Brookfield viscosity at 23.degree. C.: 620 mPa s PA1 Mean particle size: 202 nm PA1 Solids content: 55.1%. PA1 Brookfield viscosity at 23.degree. C.: 475 mPa s PA1 Mean particle size: 200 nm PA1 Solids content: 55.7%. PA1 Brookfield viscosity at 23.degree. C.: 550 mPa s PA1 Mean particle size: 195 nm PA1 Solids content: 56.0%. PA1 2-Ethylhexyl acrylate . . . 75.7 parts PA1 Vinyl acetate . . . 18 parts PA1 Acrylic acid . . . 5.3 parts PA1 Sodium vinylsulphonate . . . 1 parts PA1 Demineralized water . . . 34.7 parts PA1 Anionic surfactant marketed by Rhone-Poulenc under the name "Adex 26 S" . . . 5.71 parts PA1 Nonylphenol ethoxylated with 25 moles of ethylene oxide, marketed by Witco under the name "Rewopal HV 25" . . . 2.50 parts PA1 Brookfield viscosity at 23.degree. C.: 1500 mPa s PA1 Mean particle size: 208 nm PA1 Solids content: 56.1% PA1 Brookfield viscosity at 23.degree. C.: 1640 mPa s PA1 Mean particle size: 205 nm PA1 Solids content: 55.7%.
The bonds formed by these adhesives with the substrate are of relatively low energy and many applications require a degree of cohesion which is greater than that of the adhesion (peel, tack) to obtain an adhesive failure of the surface.
The ability of a pressure-sensitive adhesive to wet a surface well can be correlated with its glass transition temperature (Tg) and with its molecular mass. In general, obtaining a Tg lower than -25.degree. C., or even than -40.degree. C., by copolymerization of acrylic and/or vinyl monomers in the presence of transfer agents allows the desired level of tack to be obtained.
However, the influence of the Tg and of the molecular mass on the adhesive properties run counter to their influence on cohesion.
To improve the cohesive strength of pressure-sensitive adhesives without impairing the adhesive properties, a very slight crosslinking is often employed by the use of multifunctional monomers, as described in "Recent developments in acrylic polymers for latex adhesives" RAPRA 1981, Volume 18, No. 5, Abst. 7761431c.
The incorporation of chelating monomers has also been described in international application WO 91/02759, in order to increase the cohesion of pressure-sensitive adhesives after formulation of the latter with aluminium acetate.
Post-addition of such additives (Zn(Ac).sub.2, Al(Ac).sub.3) is furthermore often employed to increase the degree of cohesion of polymers containing carboxylic functional groups. However, the use of highly carboxylated latices presents problems of synthesis and of application. In fact, copolymerization of acrylic and methacrylic acid monomers in a weight proportion of polymer higher than 6% gives rise to instability problems and entails a drop in the peel strength. Furthermore, the large difference in Tg between the acids and their salts makes the Tg of the resulting composition highly sensitive to pH variations.
The search for a good adhesion/cohesion compromise therefore continues to remain one of the preoccupations of the formulators. In this context any solution that makes it possible to improve the cohesion of a pressure-sensitive adhesive without resulting in a fall in the adhesive properties, and at a cost which is compatible with the application, offers an obvious advantage.